Frequency and Duration of Bankfull Flow and Application for Natural Channel Design

The use of the 1.5 year flood (annual maximum series) as the dominant, design discharge for stabilising and restoring channels is not supported by field data. For stable channels, bankfull return periods are not uniquely designed by this value, principally because the annual maximum series ignores lesser floods which transport sediment and cause morphological change as well as the duration of transport events. Given the close correspondence between bankfull flow and the effective discharge, defined as the flow that transports most sediment in the longer term derived from daily average flow data and associated sediment transport rates, it follows that bankfull flow will be more closely associated with daily flow exceedance durations than flood return periods as it accounts for both the magnitude and duration of all transport events. Its value varies between river environments reflecting regional differences in flow and sediment transport regimes.